The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is non-uniform polishing. Due to a variety of factors, some portions of the substrate tend to be polished at a different rate than other parts of the substrate. This non-uniform polishing can occur even if a uniform pressure is applied to the backside of the substrate. In addition, a substrate arriving at the polishing apparatus may have an initial thickness that is non-uniform. Therefore it is desireable to provide a carrier head that can apply different pressures to different regions of the substrate during chemical mechanical polishing to compensate for non-uniform polishing rates or for non-uniform initial thickness of the substrate.
In one aspect, the invention is directed to a carrier head that has a housing, an upper flexible membrane coupled to the housing to define an upper pressurizable chamber, and a pressure distribution assembly positioned below the upper flexible membrane. The pressure distribution assembly transfers pressure from a portion of the upper flexible membrane to a more concentrated region of a substrate.
Implementations of the invention may include one or more of the following features. The pressure distribution assembly may include a disk-shaped plate, an annular ring, or a disk-shaped plate and an annular ring surrounding the disk-shaped plate. The carrier head may include a lower flexible membrane having a substrate mounting surface. Both the upper flexible membrane and the pressure distribution assembly may contact an upper surface of the lower flexible membrane. The pressure distribution assembly may include a rigid member to contact the upper flexible membrane and a cushion positioned below the rigid member. The surface area of the lower surface of the cushion may be less than the surface area of the upper surface of the rigid member. The carrier head may include an edge load ring to contact a perimeter portion of the substrate.
In another aspect, the invention may be directed to a carrier head that has a housing, a lower flexible membrane coupled to the housing to define a first chamber, an upper flexible membrane coupled to the housing to define a second chamber, and a pressure distribution assembly positioned between the upper flexible membrane and the lower flexible membrane. A lower surface of the lower flexible membrane provides a substrate mounting surface, and a portion of the upper flexible membrane is biasable into contact with an upper surface of the lower flexible membrane. The pressure distribution assembly includes an upper surface in contact with the upper flexible membrane and a lower surface in contact with the lower flexible membrane.
Implementations of the invention may include one or more of the following features. The pressure distribution assembly may be configured to transfer pressure from a portion of the upper flexible membrane to a more concentrated region of the lower flexible membrane. The pressure distribution assembly may includes a disk-shaped plate, an annular ring, or a disk-shaped plate and an annular ring surrounding the disk-shaped plate. The pressure distribution assembly may include a rigid member to contact the upper flexible membrane, and a cushion positioned below the rigid member to contact the lower flexible membrane. The carrier head may include an edge load ring to contact a perimeter portion of the substrate.
Potential advantages of implementations of the invention may include the following. The distribution of pressure on the substrate may be controlled. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. The carrier head may be able to either increase or decrease the pressure at the substrate center relative to the pressure on other portion of the substrate. Non-uniform polishing of the substrate may be reduced, and the resulting flatness and finish of the substrate may be improved. The carrier head may be useful in a variety of polishing procedures.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.